Chemical and enzymatic modifications of eukaryotic and bacterial cytochromes c are being used to explore the molecular basis for the specificity of cytochrome electron transport. Eukaryotic cytochrome has been specifically modified at the partially exposed heme side chain propionic acid of pyrrole III. The spectral, thermodynamic, and kinetic properties of this derivative are being explored. Limited enzymatic proteolysis is being used to remove the acid-rich terminal 25 residues of the bacterial Paracoccus cytochrome c 550 and the product is undergoing full kinetic characterization.